1. Field of the Invention
The present invention relates to a depth measurement apparatus for measuring the depth of a recessed portion such as a hole or a step, and more particularly relates to a depth measurement apparatus and a depth measurement method capable of measuring the depth of the hole having high aspect ratio with high resolution.
The present invention relates to a depth measurement apparatus for outputting 2-dimensional image information of a sample surface and depth information of the recess formed on the sample surface.
2. Description of the Related Art
With miniaturization of semiconductor devices, an aspect ratio of a recessed portion such as a via hole formed on a semiconductor wafer is higher greatly, it is strongly demanded to develop a depth measurement apparatus capable of measuring the depth of a fine via hole with high resolution. For example, with respect to the semiconductor devices currently developed, it is required to measure a via hole in a silicon substrate having a diameter of 30 μm and a depth of 100 μm with resolution of 0.1 μm. As a method of measuring the depth of such the fine recessed portion, it is considered to use a light beam for measurement. However, in the case of measuring the hole depth having a high aspect ratio, an objective lens with high NA cannot be used. Therefore, the depth of the fine hole cannot be measured using the features of confocal optical system. In addition, some via holes do not have an etching stopper formed on the bottom surface and some via holes are formed by laser processing. Such via holes do not have a flat bottom surface and thus the reflected light from the bottom surface of the recessed portion is weak. In fact, the reflected light from the bottom surface cannot be accurately measured.
As a surface profile measurement apparatus for measuring a sample surface profile, there has been known an apparatus which projects an illumination light emitted from a white light source through an objective lens onto a sample surface, captures images of an interference stripe formed by an interference of a reference light and a measurement light by a CCD camera, and measures the sample surface profile (for example, see Japanese Patent Laid-Open No. 2001-66122). According to the known surface profile measurement apparatus, an optical path length difference continuously changing is introduced between an optical path length of the reference light and an optical path length of the measurement light, a large number of images of the interference pattern are captured, and a characteristic function having a peak position matching with a peak position of the interference light is estimated. Then, the sample surface profile is measured based on the height of the peak position of the characteristic function.
What is needed is to develop a depth measurement apparatus capable of measuring the depth of a fine via hole having a high aspect ratio with high resolution, but in fact, no practical apparatus has not been developed yet. For that reason, in order to know the depth of the via hole, the silicon wafer is broken to measure the depth of the via hole. According to the surface profile measurement apparatus of the above Japanese Patent Laid-Open No. 2001-66122, the surface profile is measured from an interference pattern formed by the interference between the reference light and the measurement light, and thus, measurement with a high resolution is expected thereon. In fact, it is possible to measure the sample surface profile, but it is extremely difficult to measure the depth of the fine via hole having a high aspect ratio.
It is an object of the present invention to provide a depth measurement apparatus and a depth measurement method capable of measuring the depth of the via hole having a high aspect ratio with high resolution.